The layered compound KAlF 4 undergoes a structural phase transition in the vicinity of 250 K. Both phases have been studied by X-ray and electron diffraction and by profile refinement of the neutron powder diffraction patterns. The room temperature structure (tetragonal space group D 4h 5 −P4/mbm−Z=2; a=b=5.045 A, c=6.159 A) derived from the TlAlF 4 type is confirmed. We have determined the low temperature structure (monoclinic, P2 1 /m, Z=4; a m =7.340 A, b m =7.237 A, c m =6.407 A, β=106.8° at 4 K) closely related to the KFeF 4 structure. From structural argument it is shown that this first order transition is mainly characterized by a gliding of the successive sheets in the [100] direction. This can explain the 16° misorientation of twinned microcrystals resulting from the transition as shown by X-ray diffraction and by electron microscopy Le compose KAlF 4 subit une transition de phase structurale au voisinage de 250 K. Ces deux phases sont etudiees par diffraction RX, d'electrons et de neutrons. La transition est principalement caracterisee par un glissement dans la direction [100] de feuillets successifs ce qui permet d'expliquer apres transition la desorientation de 16° entre les microcristaux mâcles